What Materials Can Be Used For Concrete Formwork?
What Is Concrete Formwork?
Concrete formwork is a structure used in construction to hold wet concrete in place until it hardens into the desired shape for a project. It can be temporary or permanent, with temporary formwork being more common and able to be reused or resold after a project is completed.
Temporary formwork can be made of various materials, such as aluminum, steel, plastic, plywood, or timber, while permanent formwork is often made of foam.
The process of using formwork to shape concrete is known as “shuttering,” and it is a highly precise and efficient method compared to other options for setting concrete.
What Materials Can Be Used For Concrete Formwork?
Different materials that can be used for formwork in construction projects include timber, plywood, steel, aluminum, plastic, fabric, and stay-in-place or permanent insulated formwork.
Timber is lightweight and easy to work with, but not suitable for large projects or when flexible formwork is needed.
Plywood is a manufactured wooden material that is durable and lightweight, mostly used in conjunction with timber for formwork systems. Steel and aluminum are stronger and longer lasting, but also more expensive, and are often used for creating a smooth finish on concrete.
Plastic formwork is made up of modular or interlocking components and is best for smaller projects with repetitive formwork needs.
Fabric formwork uses lightweight, high-strength fabric sheets for flexible formwork, and is becoming popular for creating unusual shapes and architectural details.
Stay-in-place formwork remains in place to provide reinforcement and protect against corrosion and other damage, and is typically used for piers and columns.
Permanent insulated formwork remains in place and provides insulation, as well as other benefits such as fireproofing and sound dampening.
How Do You Calculate Concrete Pressure On Formwork?
Following are the various types of loads and pressures act on concrete formwork:
1. Vertical Loads on Concrete Formwork
Vertical loads, which can include the weight of the formwork itself, reinforcement, fresh concrete, and any additional live loads such as workers and equipment, are applied to formwork during concrete construction.
The American Concrete Institute’s Guide to Formwork of Concrete recommends a minimum live load of 2.4 kPa for horizontal formwork and 3.6 kPa for formwork using motorized carts.
The combined live and dead load design should be at least 4.8 kPa or 6 kPa if motorized carts are used.
The weight of the formwork itself is calculated by determining the unit weight of its various parts and should be considered when designing the formwork, although it is generally much lighter than the dead load of the fresh concrete and the live load of construction activity.
An allowance for the weight of the formwork, known as the superimposed load, is typically assumed to be between 0.239-0.718 kPa based on experience and then checked after the member has been sized.
This assumption is based on the fact that the typical weight of formwork falls within this range.
2. Lateral Pressure on Concrete Formwork
The internal pressure that builds up in freshly poured concrete when it is placed in vertical formwork, such as walls and columns, is caused by the weight of the concrete.
During vibration and for a short time after the vibration stops, the fresh concrete near the top of the formwork behaves like a liquid and exerts lateral pressure on the formwork that is equal to the weight of the liquid.
However, there are factors that can affect the lateral pressure of the concrete below the vibration-controlled depth, such as the rate at which the concrete is placed, the temperature of the concrete, and the internal friction of the concrete mixture.
When the concrete is placed slowly, it has time to start stiffening before the lateral pressure reaches its peak.
Additionally, the time it takes for the concrete to start setting can be affected by the temperature of the concrete and other factors such as the movement of pore water, the creation of friction, and other variables.
The type of cement, admixtures, and cement substitutes used, as well as the construction practices, can also influence the level of lateral pressure.
In general, the lateral pressure of the concrete is highest near the top and decreases as it moves down.
For design purposes, it is generally recommended to use a uniform, conservative value for the ultimate lateral pressure of the concrete.
Calculation of Lateral Pressure on Concrete Formwork
The American Concrete Institute (ACI) 347-04 specification provides guidelines for calculating the lateral pressure of concrete on formwork.
The lateral pressure is the force exerted by the concrete on the sides of the formwork. The calculation depends on the slump value of the fresh concrete and the depth of placement.
If the slump value is greater than 175 mm and the concrete is not placed with normal internal vibration to a depth of 1.2 m or less, the lateral pressure is calculated using a specific equation.
If the slump value is no larger than 175 mm and the concrete is placed with normal vibration to a depth of 1.2 m or less, the lateral pressure is calculated using a different equation for columns and walls.
The lateral pressure for walls is further divided into different categories based on the placement rate and placement height. ]
Minimum values for the lateral pressure are provided in the specification, and the unit weight coefficient and coefficient of chemistry must be taken into consideration in the calculation.
How Long Should You Leave Formwork On Concrete?
Formwork for vertical walls and columns can usually be removed after 24 to 48 hours. Slabs supported by props can usually have the props removed after 3 to 4 days, or after one week if the props are left under the soffits.
For slabs under 15 feet in height, props can generally be removed after one week, while formwork for soffits and props supporting slabs over 15 feet should be left in place for two weeks.
Props attached to beams and arches under 20 feet can be removed after two weeks, while those supporting beams and arches over 20 feet should be left in place for three weeks.
When using Portland Pozzolana Cement (PPC), formwork can generally be removed 10-7 days earlier than the above times.
It is important to ensure that the concrete is poured at the proper temperature and is properly cured to avoid cracking.
Curing can be achieved through constantly keeping the concrete wet, protecting the surface from evaporation, or applying a chemical compound.